A large complex molecule made up of one or more chains of amino acids. A typical protein contains 200-300 amino acids but some are much smaller and some much larger, e.g., titin, a protein found in skeletal muscle contains approximately 27,000 amino acids in a single chain. Proteins perform a wide variety of essential activities in cells: they largely form the physical structure of cells and cellular matrices; catalysts for all biochemical reactions are enzymes, which contain protein; the transport of materials in body fluids depends of proteins; the receptors for hormones and other signaling molecules are proteins; motion and locomotion of cells and organisms depends on contractile proteins; the transcription factors that turn genes on and off are proteins; proteins are an essential nutrient for heterotrophs; and many more - the activities of cells and organisms are largely reflections of the activities of their proteins.